Fix sphinx url

When I was fixing urls the other day, I noticed that sphinx hadn't added
https but there was an open ticket: sphinx-doc/sphinx#5522

Now that that is resolved, I'm updating it here.

docs/Makefile

@@ -9,7 +9,7 @@ BUILDDIR      = _build
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
-$(error The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the '$(SPHINXBUILD)' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/)
+$(error The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the '$(SPHINXBUILD)' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from https://www.sphinx-doc.org/)
 endif
 
 # Internal variables.

docs/make.bat

@@ -56,7 +56,7 @@ if errorlevel 9009 (
 	echo.may add the Sphinx directory to PATH.
 	echo.
 	echo.If you don't have Sphinx installed, grab it from
-	echo.http://sphinx-doc.org/
+	echo.https://www.sphinx-doc.org/
 	exit /b 1
 )